Steel sheets obtained by subjecting surfaces of galvanized steel sheets to a chromate treatment with a treatment solution mainly containing chromic acid, dichromic acid, or a salt of the foregoing for the purpose of enhancing corrosion resistance (white-rust resistance and red-rust resistance) are widely used as steel sheets for household electrical appliances, steel sheets for building materials, and steel sheets for automobiles. The chromate treatment provides excellent corrosion resistance and is an economical treatment that can be relatively easily performed.
The chromate treatment employs hexavalent chromium, which is a regulated substance causing pollution. However, the chromate treatment employs hexavalent chromium within a closed system and completely reduces and collects hexavalent chromium without releasing it into the nature. In addition, organic films can provide a sealing function of substantially preventing release of chromium from chromate films. Accordingly, the chromate treatment does not substantially cause contamination of the environment or human bodies with hexavalent chromium. However, due to recent global environmental issues, there has been a strong trend toward voluntary reduction in use of hexavalent chromium. In addition, to prevent environmental contamination caused by dumping of shredded dust of discarded products, there has already been a trend toward, in products, minimization of the amount of hexavalent chromium or reduction in the amount of hexavalent chromium.
Accordingly, a large number of treatment techniques for suppressing the generation of white rust on galvanized steel sheets without employing a chromate treatment, that is, chromium-free techniques, have been developed. For example, there is a technique of forming surface-treatment films on galvanized steel sheets by using an inorganic compound, an organic compound, an organic polymer material, or a composition containing the foregoing in combination. Although various advantageous techniques have been developed, with commercial use of these chromium-free techniques, objects that were not recognized and need to be achieved have been newly found.
It could therefore be helpful to enhance corrosion resistance in processed portions of surface-treated galvanized steel sheets. Surface-treated galvanized steel sheets are subjected to processing (cutting, bending, welding of parts, and painting) to be turned into products. When a surface-treated galvanized steel sheet is bent, a coated layer in the front surface of the bent portion is extended. In this case, a surface-treatment film is also extended by the bending and the surface-treatment film is damaged and the galvanized surface is exposed. Degradation of corrosion resistance in such exposed portions is problematic. In particular, unlike cupping causing local damage, bending causes continuous damage in the film and the coated layer. Accordingly, it is very difficult to provide processed portions having sufficient corrosion resistance.
It could also be helpful to ensure solvent resistance of a surface-treatment film. In the above-described processing, oil stains on the surface of the film or symbols written with a magic marker are sometimes wiped off with a solvent. In these cases, use of the solvent often causes a phenomenon where the surface-treatment film becomes separated and discolored to have a pale color (whitening). When a surface-treatment film is separated from a (surface-treated) galvanized steel sheet, the steel sheet does not have corrosion resistance. When a surface-treatment film turns white, the appearance quality is degraded.
It could further be helpful to ensure paintability of a surface-treatment film. There are cases where the surfaces of a surface-treated galvanized steel sheet that is processed as described above are cleaned with an alkaline cleaning agent or the like (alkaline degreasing) and the cleaned surfaces are painted. Accordingly, overpaintability of surfaces having been subjected to alkaline degreasing is sometimes required. However, there are no publicly known references regarding studies on such a characteristic.
It could still further be helpful to achieve sufficient corrosion resistance of a surface-treatment film and sufficient storage stability of a surface-treatment agent. Recent chromium-free techniques most commonly employ formation of a surface-treatment film on a galvanized steel sheet by application and drying, that is, application formation. To impart a barrier effect to a surface-treatment film formed in such a manner, the surface-treatment film needs to have predetermined water resistance. This is because it is difficult for a surface-treatment film that readily dissolves back into water to have corrosion resistance (barrier effect). In addition, in industry, it is important that a surface-treatment agent prepared as a one-component agent by mixing raw materials with predetermined proportions can be stored with stability. Considering summer, it is desirable that a surface-treatment agent can be stored with stability without alteration for a long period of time at 35° C. to 40° C. To achieve such storage stability, it is necessary that an increase in viscosity, gelation, precipitation, and the like do not occur in a surface-treatment agent, that is, the surface-treatment agent has predetermined water solubility; and the surface-treatment agent can maintain the quality at the time of the preparation thereof even after storage over a long period of time.
As described above, surface-treated galvanized steel sheets having more excellent characteristics are demanded. Hereafter, specific examples of existing chromium-free techniques will be described. Japanese Unexamined Patent Application Publication No. 53-121034 discloses a method in which an aqueous solution containing a water-dispersible silica, an alkyd resin, and a trialkoxysilane compound is applied to a metal surface and dried to thereby form a coating film. Japanese Examined Patent Application Publication No. 57-44751 and Japanese Unexamined Patent Application Publication No. 1-177380 disclose a surface-treatment method employing a water-soluble resin composed of a hydroxypyrone compound derivative to impart corrosion resistance to metal materials; and a method employing an aqueous solution of a hydroxystyrene compound or a water-dispersible polymer to impart corrosion resistance to metal materials. Japanese Unexamined Patent Application Publication No. 11-310757 discloses a technique employing a surface-treatment agent obtained by mixing an aqueous resin, colloidal silica, and ammonium vanadate with specific proportions. However, these techniques cannot achieve a film that provides corrosion resistance that is sufficient to replace chromate films.
Japanese Unexamined Patent Application Publication No. 2000-248369 discloses a technique of a surface-treatment film containing an organic resin and a thiocarbonyl-group-containing compound. However, the surface-treatment film does not provide sufficient corrosion resistance after being subjected to alkaline degreasing. Japanese Unexamined Patent Application Publication No. 11-58599 discloses a technique in which a surface of a metal plate is treated with a treatment solution that is an aqueous solution of lithium silicate containing an organic resin, a silane coupling agent, and a solid lubricant. However, the inorganic components tend to form a rigid polymer. Hence, corrosion resistance in portions processed by bending or the like is insufficient. In addition, due to the presence of the alkali metal, secondary adhesion of painting is poor. Japanese Unexamined Patent Application Publication No. 2006-43913 discloses a technique of forming a resin film with a resin aqueous solution containing a carboxyl-group-containing polyurethane resin, an aqueous dispersion of an ethylene-unsaturated carboxylic acid copolymer, silica particles, and a silane coupling agent with specific proportions. However, solvent resistance and corrosion resistance in processed portions are not sufficiently achieved. Japanese Patent No. 3573307 discloses a steel sheet having a film containing a urethane resin, a lubricant, an inorganic colloid compound, and a silane coupling agent with specific proportions. This film is designed for electrodeposition and excellent in terms of electrodeposition properties, but does not provide sufficient corrosion resistance in processed portions.
Japanese Unexamined Patent Application Publication No. 2001-59184 discloses a surface-treatment solution containing a silane coupling agent and a urethane resin and adjusted to have a pH of 2.5 to 4.5. However, the solution does not provide sufficient corrosion resistance after alkaline degreasing and does not provide sufficient solvent resistance. Japanese Unexamined Patent Application Publication No. 2003-155451 discloses a technique of forming a film with a treatment solution containing a water-dispersible resin, silica particles, and organic titanate with specific proportions. However, the technique does not provide sufficient corrosion resistance in processed portions. Japanese Unexamined Patent Application Publication No. 2006-82365 and Japanese Unexamined Patent Application Publication No. 2001-181860 disclose techniques of forming a film with a treatment solution containing an aqueous dispersion of a specific epoxy resin, a dispersion of a urethane resin, a silane coupling agent, phosphoric acid and/or a phosphate compound, and a compound having 1 to 5 fluorine atoms in a molecule. However, the film has insufficient alkali resistance and it is difficult for the film to have sufficient corrosion resistance and paintability after alkaline degreasing; and the film does not provide sufficient corrosion resistance in processed portions or sufficient solvent resistance.
Japanese Unexamined Patent Application Publication No. 2001-181860 discloses a technique of forming a film with a treatment solution containing a specific resin compound, a vanadium compound, and a metal compound containing a specific metal. However, the film has insufficient alkali resistance and does not provide sufficient corrosion resistance after alkaline degreasing; and the film has unresolved problems that, for example, the film tends to turn yellow under heating. Japanese Patent No. 3,883,831 discloses a technique of forming a film with a treatment agent containing a specific resin compound, a cationic urethane resin having a cationic functional group, a silane coupling agent having a reactive functional group, a Ti compound, and an acid compound with specific proportions. This technique provides a film excellent in terms of corrosion resistance and fingerprint resistance. However, corrosion resistance after alkaline degreasing, corrosion resistance in processed portions, and solvent resistance are not studied and these characteristics are not sufficiently achieved. Japanese Patent No. 4,078,044 discloses a technique relating to a surface-treatment agent containing at least one aqueous resin selected from a cationic resin and a nonionic resin, a metal compound containing a specific metal, and water. Similarly, alkali resistance, corrosion resistance in processed portions, and solvent resistance are not studied and these characteristics are not sufficiently achieved. Japanese Unexamined Patent Application Publication No. 2006-152436 discloses a technique employing a surface-treatment agent containing a cationic urethane, a cationic phenol polycondensate, and a compound containing titanium and a specific metal with specific proportions. However, solvent resistance and paintability are not studied and these characteristics are not sufficiently achieved.
In addition, all the existing surface-treatment agents containing silane coupling agents tend to have poor storage stability. Accordingly, even when surface-treatment agents after storage have normal appearance, the surface-treatment agents do not often provide desired characteristics. Existing techniques do not study such a problem at all.
It could therefore be helpful to provide a chromium-free surface-treatment agent used for coated steel sheets, overcomes the above-described problems of existing techniques, allows for formation of films having excellent corrosion resistance on surfaces of metal materials, provides excellent corrosion resistance in bent portions, solvent resistance, and paintability after alkaline degreasing, and has excellent storage stability in the state of a one-component agent; a method for producing a coated steel sheet with the treatment agent; and a surface-treated coated steel sheet.